Spacing pregnancies too closely or too far apart can increase the risk of premature birth for the second baby, suggests a study.

An interval of less than 18 months and more than 59 months - just under five years - increases the risk, Colombian researchers have said.

The researchers, led by Agustin Conde-Agudelo, from the Santa Fe de Bogotá Foundation reviewed 67 previous studies conducted over the past four decades, including more than 11 million pregnancies.

They said that women should use contraception and breastfeed to avoid becoming pregnant too soon after giving birth. However, the delay should not be too far apart.

According to the study, it was best to leave between two and five years between pregnancies, the online edition of BBC News reported.

More than one million children worldwide are thought to die each year in the first four weeks of life as a direct result of pre-term birth.

Infants born to women who conceived within six months of their previous birth have a 40 percent increased risk of pre-term birth, the researchers found.

They also found that pregnancies spaced longer than five years have a significantly higher risk of problems - including premature birth and low birth weight - than those spaced 18 to 23 months apart.

Women who have just had a baby have temporarily diminished nutritional stores, especially if they breastfeed or have low levels of essential vitamins. This could reduce the ability of mother to support a foetus conceived shortly after a birth.